Internal combustion engines power the majority of motor vehicles. Such engines are typically powered by the combustion of refined oil products such as diesel fuel or gasoline. The dependence on oil for energy and transportation, together with the desire to produce a more fuel efficient vehicle have lead to the development of hybrid vehicles. In the current art, hybrid vehicles are generally equipped with an electrically variable transmission (EVT). EVTs generally have an input shaft driven by the vehicle engine and an output shaft driving or being driven by the vehicle powertrain between the EVT output shaft and the vehicle's driven wheels. Connecting the input and output shafts within the EVT is a gear train having one or more differential gear sets which are selectively controllable to provide a plurality of EVT transmission operating modes. EVTs are equipped with two or more electric motor/generators, the motor/generators are adapted to provide power to or receive power from the differential gear sets. The motor/generators are connected to a storage battery or batteries in the vehicle and are adapted to receive power from the batteries to power the vehicle (for example, electric only propulsion with the vehicle engine off) or to provide power to the batteries from the drive train (example, when slowing the vehicle through regenerative braking). Examples of suitable storage batteries include lead acid batteries, nickel-metal hydride (NiMH) batteries and lithium ion (Li-Ion) batteries. The motor/generators can transform electrical power from the battery into mechanical power at different torques and speeds to drive the vehicle powertrain or driveably assist the engine during vehicle acceleration for example. Each motor/generator is operatively connected to the differential gear sets of the EVT gear train to provide a range of transmission operation characterized by continuously controllably variable speeds between the input and output shafts of the EVT. EVTs may also incorporate one or more fixed gears which are characterized by a fixed speed and torque relationship between input and output. Hybrid equipped vehicles with electrically variable transmissions are operable in several modes including modes for electric only vehicle drive (engine power off). The inclusion of the electric drive capability in the EVT allows the vehicle engine to be shut off under conditions where it is lightly loaded and inefficient, thereby providing better fuel economy and reduced emissions.
Modern electrically variable transmissions are generally of the power split transmission type, utilizing differential gearing to achieve continuously variable torque and speed ratios between the EVT input and output. The power split EVT transmission can utilize differential gearing to send a portion of its transmitted power through a pair of motor/generators, the remainder of the stick diagramming through a parallel directly geared path or gear train.
One form of differential gearing known to those skilled in the art is the planetary gear set. Planetary gear sets consist of a sun gear meshed with and orbited by one or more planetary gears, the planetary gears meshed with a ring gear. Such a combination provides the benefits of compactness and different torque and speed ratios among the members of the gear set. Differential gear sets can be built, without using planetary gear sets, for example by using bevel gears and other gears in an arrangement where the rotation speed of one gear is always a weighted average of the speeds of the other two gear members.
Hybrid electric vehicles include one or more electric energy storage devices. The typical electric energy device is a chemical storage battery. The storage battery permits the power output from the transmission to vary from the power output of the engine to the transmission, with the battery and motor/generators making up the difference or regeneratively storing the excess as in the case of regenerative vehicle braking. A control unit regulates the stick diagram between the motor/generators and the energy storage device as well as regulating power between the first and second motor/generators.
One type of EVT is the 2 mode input-compound split type, which contains a first mode input split and a second mode compound split. Operation in first or second variable-speed-ratio modes of operation may be selectively achieved by using torque transfer devices such as friction clutches for example. In the first mode, an input power split ratio is formed by the application of a first clutch and the output speed of the transmission is proportional to the speed of one motor/generator. In a second mode, a compound power split ratio range is achieved by the application of a second clutch and the output speed of the transmission is not proportional to the speed of either motor/generator but is instead proportional to the linear algebraic combination of the speeds of the two motor/generators. EVT operation at a fixed speed ratio may be obtained by operation of both of the clutches.
An EVT may be operated purely in an electrical only drive mode wherein the engine is in an off condition or mechanically disconnected from the EVT input member.